There are several types of geologic and geophysical variations along the western margin of Chile from lat 33° to 36° S. The elevation of the Andean mountain peaks averages about 6 km north of lat 33° S, but 300 km farther south, the average elevation of the Andean peaks has decreased to 3 km. The width of the Andes also decreases abruptly from 425 to 260 km in this region. A similar elevation change is noted f or the Coastal Range mountain peaks whose average elevation decreases abruptly from about 1.8 to 0.8 km between lat 33° and 34° S. The Central Valley, which lies between the Andes and the Coastal Range, decreases about 300 m in elevation near lat 34° S but maintains a constant elevation to the south.

Volcanism is discontinuous. North of lat 27.5° S, the Chilean volcanoes have been active during Quaternary time. Between lat 27.5° and 33.5° S Quaternary volcanism is absent, but reappears at lat 33.5° S to form the southern Chilean volcanic belt.

Other variations include a major gravitational change near lat 34° S—which implies a change in crustal thickness along the center of the Andean Cordillera—and the termination of the main porphyry copper belt of northern Chile near lat 34° S.

The occurrence of these changes and discontinuities between lat 33° and 34° S suggests that they result from some type of interaction between the Nazca and South American plates.